TIDES Flashcards
what are tides?
- waves
- exact form of tidal record varies with location
how are tides created?
- gravitational attractive force acts as centripetal force tht keeps earth and moon in mutual orbit around common centre of mass
does the magnitude of force vary with distance?
yes
consider magnitude of force for unit mass at three locations?
- on earths surface directly beneath moon
- at centre of earth
- on earths surface on opposite side to moon
what does the equilibrium tidal theory consider?
- spherical earth covered with uniform ocean
- action of gravitational force on fluid elements in ocean
what is tidal generating force (TGF)?
- net force acting on a fluid element
how to calculate TGF?
mass of tide raising body/ distance to tide raising body ^3
how are tidal bulges formed?
- TGF causes movement of water
- two bulges of water on opposite sides of the earth
- aligned with the tide raising body
how are diurnal/ semidiurnal tides created?
- earth spins on axis once per day
- points on the surface move beneath tidal bulges
- form of tide depends on latitude and lunar declination
what is lunar declination?
- angle of moons orbit with equator
tidal periods?
suggested = 24 HOURS (D) 12 HOURS (SD)
OBSERVED = 25H50M (D) 12H25M (SD)
what tide does moon affect?
lunar tide
what tide does sun affect?
solar tide
what is the spring neap cycle?
- considers co- occurrence of lunar and solar tides
- tow sets of tidal bulges. one with moon one with sun
- overall tide is superposition of both tides = depends on alignment of earth, moon and sun
timing of spring neap cycle?
- fortnight cycle of tidal variations
spring tides simplified
- sun earth and moon make straight line in space
- new moon or full moon
- tidal bulges add together
- higher HW and lower LW
= increased tidal range
neap tides simplified?
- sun, earth, and moon make right angle in space
- first quarter or third quarter
- third bulges work in opposition
- lower HW and higher LW
- decreased tidal range
factors of equilibrium tidal theory?
- explains origin of tides
- correctly predicts key features
- cannot capture details of real tides
factors of dynamic tidal theory?
- constraining influence of land masses
- frictional influence of rotating earth
- deflection of flows by coriolis effect
what is the rotary tidal systems?
ASSUME CLOUD BASIN IN NORTHERN HEMISPHERE
- water flowing in basin deflects to right and hugs coast
HW travels counter- clockwise around basin
- maximum tidal range at coast
- minimum tidal range in centre
= rotary wave (kelvin wave)
= amphidromic system
what can amphidromic systems do?
- create maps summarising tidal variations in a region
how do you create a map summarising tidal variations?
- draw lines joining locations of equal tidal phase
- where HW occurs at the same time
- COTIDAL or COPHASE lines - draw lines joining locations of equal tidal range
- CORANGE lines
what is the central (nodal) point called?
- amphidromic point
